Known ploughs for this purpose act by simple wedging into the ballast, due to the forward motion of the machine to which the plough is attached, and undergo a very strong drag, either because the gravel of the old ballast has become compact due to the ramming action exerted by the train traffic and to the bonding action of mud, metal powders and other substances deposited upon the ballast, or because the low operating speed of the tie tamper does not promote the development of a regular flow of gravel displaced by the plough. Consequently, it is not possible to make with such ploughs a cutting of substantial depth. In practice, one must limit oneself to equalizing the ballast by pouring into the furrows left by the removed ties the gravel previously located between adjacent ties, while pushing laterally only a small amount of gravel. The new ties are thus laid at a level higher than necessary and a special machine must subsequently be used for excavating the gravel under the new ties in order to lower their level to the extent just necessary for restoring the proper laying level after the packing operation causing the ties to be raised a few centimeters. In many cases it is required that the level for laying the new ties be lower than that of the old ties, due to the adoption of higher ties and/or rails, and to the necessity of keeping the track plane at the same previous level, subordinate to the height of the overhead line, to the tunnel gage, to level crossings, and the like.